Process for making high quality steels

ABSTRACT

HIGH QUALITY STEEL IS MADE BY REDUCING THE SULPHUR AND NITROGEN CONTENTS OF A MOLTEN CHARGE OF IRON IN AN AGITATOR LADE, THEN REFINING THE MELT IN AN ACID PROCESS SIDE-BLOW BESSEMER CONVERTER AND SIMULTANEOUSLY AGITATING THE MELT IN THE CONVERTER PREFERABLY BY APPLYING AN ALTERNATING MAGNETIC FIELD. THE QUALITY OF THE STEEL THEN PRODUCED APPROACHES THAT OF STEEL MADE BY THE MORE EXPANSIVE OXYGEN BLOW PROCESS.

United States Patent 01 fice 3,597,191 PROCESS FOR MAKING HIGH QUALITY STEELS Giinter Altland, Lendringsen, Kreis Iserlohn, Germany, assignor to Gesellschaft fur Hullenwerksanlagen m.b.H., Dusseldorf, and Eisenwerk Rodinghauseu, Lendringsen, Kreis Iserlohn, Germany No Drawing. Filed Nov. 13, 1967, Ser. No. 682,554 Claims priority, applicatiorg 8e5rmany, Nov. 29, 1966,

Int. Cl. cilc 5/00 US. Cl. 75-46 2 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a process for making high quality steels, using a side-blow Bessemer converter. In the operation of side-blow Bessemer converters, using an acid lining, air is blown in above the surface of the bath, that is to say above the surface of the slag, and consequently the oxygen in the air does not react directly with the melt, the reaction taking place through an intermediate slag phase. The advantage of a side-blow converter is that the melt picks up very little nitrogen from the blown air, because the slag acts as a barrier between the air and the melt. Nitrogen in the melt impairs the qualit of the steel. On the other hand, a disadvantage of the side-blow converter is that its capacity is limited to between 3 and 5 metric tons. This upper limit to the capacity of the converter is due to the fact that a large volume of melt involves a large converter cross-sectional area and a great depth of bath, and consequently an adequate rate of refining cannot be obtained. The rate of refining depends on the surface areas of the two interfaces, between the air and the slag and between the slag and the melt, and is adversely influenced by the depth of the bath in that the greater the bath depth the longer are the diffusion paths followed by the reagents. A further disadvantage of using large volumes of melt is that the blown air stream has a parabolic shape, and if the surface to be blown is large the blow cannot reach the entire surface of the bath. A great depth of bath slows down the oxidation of the non-iron constitutents of the melt, because the rate of oxidation depends on diflfusion, and this is adversely influenced by the length of the path which the reagents have to follow before they can react together.

Considerable reductions in the sulphur content of molten iron have been obtained using an agitator ladle for containing the molten iron and adding calcium carbonate and/or calcium carbide to the melt. The agitator ladle is supported by frame and is oscillated about its longitudinal axis. The agitator ladle does not, itself, rotate. As soon as the speed of the ladle reaches a certain critical value, a wave forms in the bath and produces a good mixing effect throughout the melt, which considerably increases the rate at which the refining reactions take place. It has also been found that, in addition to the removal of sulphur from the iron nitrogen is removed by up to 50% of its initial concentration.

3,597,191 Patented Aug. 3, 1971 It would be desirable to be able to manufacture highquality steels containing, in particular, extremely low concentrations of nitrogen, such as can be obtained at the present time only by means of the oxygen upward blast process, at much less cost than this process. With this aim, according to the present invention, a process for making high quality steels comprises the steps of reducing the sulphur and nitrogen content of molten iron in an agitator ladle, and refining the resulting melt in an acid process side-blow Bessemer converter, the bath in the converter being agitated to increase the rate at which refining takes place.

In a side-blow converter of large capacity it is not possible to obtain sufiicient agitation of the bath to obtain intensive mixing all the way through the bath, merely by means of the air blow, and furthermore, in contrast to the bottom-blow converter, the reaction between the atmospheric oxygen and the non-iron constituents of the melt does not take place directly but occurs through the slag phase. For these reasons a melt weighing more than 5 metric tons must be given a swirling movement and this must include the bottom layer of the melt, so as to considerably shorten the diffusion paths and thus increase the rate at which the reactions take place. This is preferably done by means of an alternating magnetic field which is generated by a polyphase coil arranged at the bottom of the converter. This produces a force acting tangentially along the bottom of the converter. The direction of the force can be adjusted as desired, so that for example the force can encompass the entire converter bottom, moving the bottom layer of the melt from one side to the other. The melt therefore rises upwards at one side of the converter, moves across under the layer of slag to the other side of the converter and then returns along a curved path near the wall of the converter and so finally reaches its starting point again. The velocity of this movement depends on the design of the coil, the thickness of the ceramic converter lining, the depth of the bath and the viscosity of the melt.

In practise, a preliminary melt of molten iron is made, either in a blast furnace or in a cupola fed exclusively with scrap. The melt is then fed to an agitator ladle of the usual kind in which sulphur and nitrogen are removed from the preliminary melt by the introduction of calcium carbonate, calcium carbide and/or sodium carbonate. After that, the melt is fed to a side-blow converter containing an acid lining. As soon as the blow begins a voltage is supplied to the coil, and very rapid oxidation of the non-iron constituents of the melt is obtained, by virtue of the swirling and mixing movement in the bath. The process is advantageous in that high quality steel is produced, having in particular a low nitrogen @55- centration, and also in that production costs are low. This is because an acid converter process is comparatively inexpensive and a refining air blow is less costly than the oxygen blow used in the oxygen upward blow process. Also, if desired, alloying substances can be introduced in the converter itself, the swirling movement ensuring rapid solution and a good distribution throughout the melt.

The constituents of three example melts at various stages through the process in accordance with the invention are listed in the tables below. In the process, an acid process side-blow converter was operated with a blow pressure of only 0.3 atmospheres gauge. The converter was fed with a melt derived from a cupola which itself had been charged with a scrap of the composition shown in Table I. I

TAB LE I Percent II III 2. 70 2. 86 I. 25 1. 0. 55 0. 55 0. 026 0. 028 0. 106 0. 120 0. 0198 0. 0165 Fe Balance Balance Balance In the agitator ladle, the sulphur and nitrogen contents of the melt were reduced by the introduction of 0.75 to 1% by Weight of calcium carbide together with about 0.5% by weight of coke fines, followed by approximately minutes shaking. The resulting melts had the composition shown in Table II.

TABLE II Percent I II III 3. l4 3. 22 3.30 1. 40 l. 23 1. 03 0. 58 0. 54 0. 50 0. 030 0. 028 0. 025 0. 018 0. 013 0. 018 N2 0. 0125 0.013 0. 0108 Fe Balance Balance Balance Table III.

TABLE III Percent Melt I II III C 0.13 0. 09 0. 12 Si Mn 0.31 0.34 0.38 0.027 0. 027 0. 030

.. Balance Balance Balance These values show that the steel made by the process according to the invention is of a quality approaching that of steel made by the oxygen blow process. This result is, however, obtained at a far lower production cost, and furthermore the volume of a batch of melt which can now be processed in a side-blow converter is no longer limited to 5 metric tons or less. The high quality of the steel made by the process according to the invention includes a considerably increased toughness, that is to say an increased elongation at rupture and an increased necking factor, compared with steels of the same yield strength and tensile strength.

I claim:

1. A process for making high quality steels comprising the steps of preparing a preliminary melt of molten iron, reducing the sulphur and nitrogen content of said molten iron in an agitator ladle, refining the resultant melt in an acid process side-blow Bessemer converter and, simultaneously with said refining step, agitating said resultant melt in said converter to increase the rate at which said refining takes place.

2. A process for making high quality steels comprising the steps of preparing melt of molten iron, reducing the sulphur and nitrogen content of said molten iron in an agitator ladle, refining the resultant melt in an acid process side-blow Bessemer converter and, simultaneously with said refining step, agitating said resultant melt in said converter to increase the rate at which said refining takes place by producing an alternating magnetic field by passing an aiternating current through a polyphase coil arranged in the bottom of said converter.

References Cited UNITED STATES PATENTS 837,598 12/1906 Weaver 46 1,839,927 9/ 1928 Neuhaoss 75-60X 1,925,247 9/ 1933 Hennig 75-46X 2,646,351 7/ 1953 Heimberg 75-46 2,802,731 8/ 1957 Heimberg et a1 75-46 2,875,036 2/1959 Kalling 75--45 2,900,249 I 8/1959 Kerlie 7560 L. DEWAYNE RUTLEDGE, Primary Examiner I. M. DAVIS, Assistant Examiner US. Cl. X.R. 75-60 

